The present invention is directed generally to electrophotographic printing, and, more particularly, to a process for printing a novel toner that contains a combination of color-sensitized silver halide crystals, developing agents, color coupling agents, and, optionally, fixing agents.
Color photography is a well-developed process. However, while the resultant colors are reasonably stable, no process has been developed to print computer-generated color images. Even so-called xe2x80x9cinstantxe2x80x9d photography has not found use in printing computer-generated images, for example.
Briefly, in the general photographic process steps, light is directed to a silver halide (AgX) grain center. The silver halide crystal absorbs incoming energy and becomes excited (electron moves from the valence band to the conduction band). The silver halide crystal then draws an electron from the developer agent, e.g., p-phenylenediamine (PPD) and is therefore reduced (an electron is absorbed from PPD). The PPD developer is oxidized (due to the removal of the electron), leaving quinone-diimine (QDI). The QDI bonds with a color coupler or coupler ion to form an indoaniline or azomethine dye, depending upon chemistry choices. At the end of this reaction, dyes of the incident light color are created. It should be noted that each core containing color-sensitized silver halide crystals becomes colored to the wavelength it recognizes only, where the system or combination of cores then reproduces the incoming colors.
Any additional light will continue to be absorbed by AgX and continue reaction and creation of color dyes, ultimately creating black. Accordingly, the oxidation of PPD must be terminated at some point. This is accomplished by exciting the balance of unexcited AgX and saturating the AgX with electrons in a process known as xe2x80x9cfixingxe2x80x9d. Reactions are halted, and a stable colored state is the result.
Color printing has emerged as an alternative to conventional silver halide photography. The attraction of color printing is substantially immediate production of the desired image.
Presently, a number of approaches to color printing have been developed: (1) ink jet technology, such as Hewlett-Packard""s DeskJet printers, (2) dye sublimation, such as by thermal wax transfer, and (3) electrophotographic technology, such as Hewlett-Packard""s LaserJet printers.
Color ink jet technology involves the expulsion of droplets of different colors of ink comprising colorants, or chromophores, in a vehicle. The expulsion is either from a controlled series of heated resistors or from a controlled series of piezoelectric elements. However, the chromophores are not as stable, or colorfast, as one would like, and images tend to fade over time. Further, waterfastness, smearfastness, and UV-fastness problems continue to be the focus of efforts of continuing improvements. While progress is being made, work continues in these areas.
An advantage of the dye sublimation process is that the dot intensity at a given location can be varied, without having to employ different inks (as in color ink jet printing) or without the lack of variation (as in color laser jet printing). However, the resolution is fairly limited, 300 dots per inch (dpi), for example.
Color electrophotographic technology employs color toners is an alternative technology to color ink jet technology. The former tends to be more expensive initially (in terms of printer cost), but less expensive over the long term, and in any event is faster. Further, colorfastness and image fade are not major issues as they are with ink jet printing. Nevertheless, typical electrophotographic processing requires multiple process steps and complex toner mixing. Further, typically, three colors of toners are used to produce the spectrum of colors. Each toner is applied with a separate EP process. The colors to be exhibited require careful application of each toner type to provide acceptable colors, Separate toner is used to provide monochrome black.
Briefly, in the general electrophotographic process steps, a laser (or other means, such as a light emitting diode) shines energy to a finite area on the electrophotographic drum, exciting the finite area on the drum coating. The drum is coated with a photovoltaic material that retains a charge once excited. Toner is stored in a toner cartridge hopper. The toner is agitated and electrostatically charged. A field generated by voltage differences propels the charged toner particles from region to region. The toner is attracted to the laser-charged finite area on the surface of the EP drum. The toner attached to the drum rotates toward a nip. A print medium is transported through the nip. The opposing roller in the nip is charged to attract the toner towards its surface and away from the EP drum. The print medium, located in the nip between the two rollers, is the receiver of the transported toner. As the print medium leaves the nip, the toner remains in place on the surface of the print medium. Finally, the toner is fused to the print medium, typically using both heat and pressure via a roller nip. In conventional processing, the toner is a plastic material having finite melting or glass transition temperatures. Fusing is a process of melting the toner into the fibers of the print medium.
A need remains for a color printing process that retains the advantages of the foregoing prior art approaches, while overcoming most, if not all, of their drawbacks. Since a novel color toner is disclosed and claimed in the above-referenced related application, there is a need for a process that effectively prints that toner and results in color photographic-quality imaging.
In accordance with the present invention, an apparatus and process are provided for printing particles of a color toner that can be selectively colored or that comprises particles that can demonstrate or reflect color based upon exposure to light of known wavelength, intensity, and duration. One preferred embodiment of the color toner is one that contains a combination of light-sensitive oxidizing agent(s), such as color-sensitive silver halide crystals, developing agent(s), color coupling agent(s), and, optionally, fixing agent(s).
The color toner, itself translucent prior to exposure to light, is capable of providing any of a number of selected colors, the particular color realized being dependent on the particular wavelength to which the particle is exposed. The toner may alternatively comprise a single color, such as for printing blue-prints, or may comprise a mixture of particles, each containing specific color-sensitized silver halide crystals, along with the various foregoing agents.
The apparatus and process of the invention essentially combine photographic and electrophotographic processes in a novel manner.
The apparatus for printing particles of the color toner comprises:
(a) a medium transport for transporting print media through the apparatus on which the toner particles are printed;
(b) a rotatable electrophotographic element;
(c) a light source, such as a laser, for shining radiation on a finite areas of the electrophotographic element, thereby charging the finite areas on the element;
(d) a light-tight source containing a quantity of the toner particles, which are attracted to the laser-charged finite areas on the surface of the electrophotographic element as the surface of the rotating element is passed through the toner source;
(e) a transfer roller urged against the electrophotographic element with sufficient pressure to form a nip through which the print medium is transported, the transfer roller being charged to attract the toner particles away from the electrophotographic element and onto the print medium;
(f) at least one light source for selectively exposing the toner particles to light of known wavelength, intensity, and duration to initiate (1) excitation of silver halide, (2) developer reaction, and (3) color coupler reaction;
(g) a fixing mechanism for fixing unexcited silver halide by saturating the unreacted silver halide with electrons and for terminating developer and color coupler reactions; and
(h) a fuser for bonding the toner particles to the print medium.
The process of the present invention comprises:
(a) providing a print medium for transport through the apparatus of the present invention;
(b) developing an image by directing a laser or other source to shine energy to finite areas on the rotating electrophotographic element, thereby charging the finite areas on the element;
(c) passing the surface of the rotating electrophotographic element through the toner source to attract the toner particles to the laser-charged finite areas on the surface of the element;
(d) providing a charge to the transfer roller while the print medium is passed through the nip, thereby transferring the toner particles to the print medium by attracting the toner particles away from the electrophotographic element, the toner particles remaining on the print medium as the print medium leaves the nip;
(e) selectively exposing the toner particles by light of known wavelength, intensity, and duration;
(f) initiating (1) excitation of silver halide, (2) developer reaction, and (3) color coupling reaction;
(g) fixing the toner particles by (1) exciting the balance of unexcited silver halide and saturating the silver halide with electrons, (2) terminating the developer reaction, and (3) terminating the color coupling reaction; and
(h) bonding the toner particles to the print medium.
Since the above-described color toner employed in the practice of the present invention includes all necessary colors that are selectively activated, only one color toner and thus only application of one color toner is required, thereby (1) reducing the application of multiple toner colors, (2) reducing the application of multiple color inks, (3) simplifying the toner processing, since selective application of xe2x80x9ccoloringxe2x80x9d, as opposed to chemical application, is employed, and (4) providing flexibility in producing color or monochrome images. Advantageously, toner may be selectively applied to image locations only, and then selectively exposed.
The resolution of the resulting print is similar to that of conventional photography, namely, about 3,000 dpi. This is in contrast to (1) color ink jet (1,700 dpi), (2) color dye sublimation (300 dpi), and (3) color laser jet (1,200 dpi).
The process of the present invention (1) increases the robustness of color image processing and mechanically simplifies the process of application and (2) results in high quality, time-enduring images.